A production logging tool which measures the density of the production fluid in a well borehole, particular a cased well, typically funnels the fluid into the tool where density measurements are made. Many tools operate by measuring the attenuation of gamma rays emanating from a radioactive source which directs the radiation through the fluid to a detector. One disadvantage of this approach is that the well fluid is disturbed during measurement, yielding a density value that is not indicative of the fluid in the well bore. Moreover, such a measurement is only an average measurement and cannot measure asymmetries in the density distribution as a function of radial direction. The column of fluid in a well may stratify if it has been standing for any interval or is slowly flowing. Water will settle to the bottom and the oil will rise to the top. This is a problem in a slant hole, or, a highly deviated well, where the density differential between oil, water and gas can cause the fluid to stratify, with the oil and gas rising to the high side of the hole. The present disclosure is directed to a device which will measure fluid density and provide a measurement of density in all radial directions, and which will also indicate density variations along the length of the tool. The present disclosure sets out a radioactive radiation source which is a typical radioactive isotope, typically one with a long half life. One approach is to provide shielding which is so located that impinging gamma rays reaching the detector pass only through the fluid around the tool. The detector is surrounded by shielding arranged in this fashion. Accordingly, the gamma rays which impinge on the detector travel only through the fluid. When the gamma rays emitted by the source are scattered by the fluid, those deflected to the detector will provide a fairly direct measure. As a generalization, the signal is proportionate to electron density of the materials along the path of travel of the gamma rays impinging on the detector. Generally, gamma radiation through the fluid interacts with the fluid by Compton scattering. While other types of scattering are possible, the great probability is that the scattering is Compton scattering only. Since Compton scattering depends on the density of electrons in the medium, which is related to the bulk density, the extent of scattering depends on the bulk density of the medium, or the fluid which surrounds the tool. In the event, however, that the gamma rays emitted by the source enter the pipe (primarily steel) or enter the adjacent or surrounding cement and formations, there are other interactions between the gamma rays and the materials which make up the steel pipe concrete and adjacent formations. At this juncture, there will be a statistically measurable scattering of gamma rays by coherent scattering or photoelectric absorption.
If the tool is centralized, it is axiomatic that a gamma ray which passes through the well pipe must first pass through the fluid. Generally, if all of the gamma rays must traverse the fluid and very few of the gamma rays that do enter the pipe are ultimately detected, then the materials making up the pipe and surrounding structure are less important to the scattering mechanism. In that instance, the fluid density can be determined from the count rate of a single detector. So to speak, a single measurement yields a single unknown or variable referring to the electron density of the fluid and hence, the bulk density of the fluid. By contrast, if a significant number of the gamma rays that enter the pipe are scattered back to the detector, then the measured count rate to some extent depends on the absorptive properties of the pipe and the materials which are on the exterior of the pipe. In that instance, a single measurement cannot be used to provide two variables, one relating to the bulk density of the well fluid and the other relating to the bulk density of the pipe and materials beyond the pipe. In that instance, a single count rate simply will not provide sufficient data to determine two variables from one measurement. It is, however, possible to have two detectors which make two separate measurements and the two measurements can be used to determine two variables, namely, one from the fluid electron density or the bulk density of the fluid. The other measurement relates to the pipe and other confining materials beyond the pipe. A further factor in making measurements is preferably the incorporation of shielding and collimators which are axially symmetric so that resultant measurements provide an average of the fluid density fully around the tool. In other words, the radiation is transmitted from the source in all directions of azimuth. By this approach, all of the fluid which is in the borehole can be tested and data thereby obtained representative of all of the fluid.
Alternately, it is possible to collimate the source and detector so that the preferred range of illumination by the radiation source is limited to a specified azimuthal range, e.g., irradiation at an azimuth of thirty degrees width. In that instance, the tool can be used where the port is directed along a particular azimuth line, data taken at that angle, and then the tool can be rotated to other angular directions. This is particularly helpful in deviated holes where the fluid composition can vary significantly with angle. Of course, it is advantageous to use a navigational package to correlate the angular measurements to an absolute direction.
The foregoing mentions a single detector system and then a two detector system where two detectors in effect provide measurements yielding two variables. By the use of a third detector, another variable can be obtained. Because the detectors will be placed at different distances from the source, on average the gamma rays detected by a detector will have traveled farther from the tool in a radial direction than gamma rays detected by a detector closer to the source. Thus, the count rate of the detector closest to the source will be most sensitive to the fluid closest to the tool, whereas the middle detector will be more sensitive to the fluid farther from the tool. The detector farthest from the source will be most sensitive to the casing and cement. Thus, information from the three detectors can be used to determine the fluid density near the tool and the fluid density farther from the tool.
The present disclosure also contemplates the use of a detector which is capable of determining count rates in particular energy windows. For instance, in the use of a scintillator with a photomultiplier tube (hereinafter PMT), the energy spectra from the detector can be classified into specific energy windows. In general terms, the low energy gamma rays will have traveled further through the fluid than high energy gamma rays. Accordingly, the sensitivity to the surrounding media at different distances from the tool will vary with the energy window. thus, one detector could measure two count rates, one that is primarily sensitive to the fluid density, and one that has a greater sensitivity to the casing. The two count rates could be combined to provide a measure of the fluid density that is independent of the casing. Also, the different energy windows could be used with two or more detectors. The fluid density could then be determined independently for the different energy windows, yielding densities that correspond to different distances from the tool.
In general, the measurement will be improved if the tool is centralized in the hole. The centralization can be implemented with a passive device that clamps on to the tool or with a powered centralizer.
In summary, the present apparatus is a fluid density measuring system utilizing a source and preferably two or three detectors. Shielding material can be incorporated to collimate the irradiation from the source, and the detectors can likewise be collimated to receive gamma ray radiation from specific directions. Moreover, the ports or windows which emit the radiation from the source or direct radiation toward the detectors can either be 360.degree. or include lesser angles. In any event, a determination of fluid density can be derived, and to the extent that the fluid density is determined, it can be determined free of factors relating to the surrounding steel casing and other materials.